Factors influencing stranding of wild juvenile brown trout (Salmo trutta) during rapid and frequent flow decreases in an artificial stream

To provide environmental guidelines for operation of peaking hydropower plants, stranding experiments with juvenile brown trout (Salmo trutta) were conducted in a 3.8 m wide and 19.2 m long artificial stream. We found a significant decrease in stranding of trout fry by reducing the dewatering speed from >60 cm h(-1) to < 10 cm h(-1). At water temperatures around 11degreesC we found that less than 8% of the trout stranded during night experiments (on average for all subsets at dewatering rate >60cm h(-1)). However, the highest stranding rate occurred at water temperatures around 7degreesC during rapid dewatering at night (mean 22% stranding). Significantly more fry were stranded in the first versus the second to fifth dewatering episode (mean 22% versus 10% stranding) at rapid daytime dewatering in 11degreesC water. Stranding of larger fry (>0 +) was negligible at water temperatures around 11degreesC, except at rapid dewatering during daylight. Further, twice as many 0 + trout as larger fry (>70 mm in June, >60 mm in late summer) stranded during rapid dewatering during daytime, and almost three times as many 0+ stranded during rapid dewatering when they were mixed with trout parr compared to homogeneous 0+ experiments. Our study showed a tendency towards an increased stranding of fry with long habituation time (<30 hours versus >30 hours with stable flow). We recommend dewatering in darkness at all times of year to reduce stranding of salmonids, and to use slow ramping rates <10 cm h(-1.) After longer periods with stable flows, a gentle drop in discharge is recommended, which might also reduce stress and possible sub-lethal effects. Copyright (C) 2003 John Wiley Sons, Ltd.